In cacti you can get and display only "simple" data, like traffic/cpu/disk space from standalone host. "Agregate" plugin can help only in particular case, like summarize similar data.
I need more complex functionality - derivative data: data derived from formula, based on other data (with storing in rrd, or without it).

Example: I have cluster of N hosts, each host provide set of "simple" data (traffic in, traffic out, count of logged users). As a result I want to see, for example, "average traffic per logged user over the cluster" - sum(traffic)/sum(users).

gandalf wrote:next version will support COMPUTE data sources, as rrdtool calls them.

Sounds great, thx.
But COMPUTE data sources restricted by one rrd file (computation between data sources in this file). I search a way to combine data from different rrd files, with aggregate-like logic - determine graph/source to combine and formula in general view (a+b/c, where a, b, c - multiplication/summarization of similar data sources).

We try to support most/all natice rrd features. That why we chose to implement COMPUTE.
Yes, it does NOT tackle all use cases; the one you pointed out is not solved by this approach. We currently do not have an approach to provide such a feature, an that's mainly the reason why we did not put in on the feature list.
Your options:
- either provide code that does what you need
- or at least provide an approach how this feature can be implemented
Then, let's restart the discussion
R.